Reaction process for polymerization



May 15, 1962 R. A. FINDLAY REACTIQN PROCESS FOR POLYMERIZATION FiledFeb. 24, 1960 United States Patent 3,035,040 REACTION PROCESS FORPOLYMERZATON Robert A. Findlay, Bartlesville, Okla., assigner toPhillips Petroleum Company, a corporation of Delaware Filed Feb. 24,1960, Ser. No. 19,619 Claims. (Cl. Zoll-94.9)

This invention relates broadly to an improved process and arrangement ofreaction zones for the polymerization `of polymerizable hydrocarbons. Inaccordance with one aspect, this invention relates to an improvedcatalytic polymerization process wherein higher catalyst producytivityis obtained. ln accordance Wi-th another aspect, this invention relatesto an improved reactor system for the polymerization of polymerizablehydrocarbons, especially lolens.

Various processes and reaction systems have either been proposed or usedfor polymerization and other reactions in the prior art. However, manyof these processes or systems are limited in their applicability for onereason or another. For example, many prior art systems that areadaptable for one process often cannot he satisacton'ly employed inanother. Other known systems are not satisfactory for exothermicreactions, for example, polymerization and particularly inpolymerization processes where high molecular weight normally solidmaterials are produced since these systems are not ordinarily suitablyadapted to remove the heat of reaction or make the most eicient use ofthe catalyst employed. The present invention relates to an improvedreactor system or `arrangement particularly adapted to polymerizelolefins, for example to normally solid polymers, whereby increasedcatalyst productivity, improved heat exchange, and other advantages `areobtainable.

Accordingly, an object of this invention is to provide -a novelarrangement of reaction zones for polymerization.

Another object of this invention is to provide an irnproved processwherein provision is made for maintaining close control over reactiontemperature.

Another object of this invention is to provide a multistage process forpolymerization wherein close control of polymerization temperature ismaintained.

A further object of this invention is to provide a multistagepolymerization process wherein increased catalyst productivity isobtainable.

Other aspects, objects, as well as the several advantages will becomeapparent upon a study of the disclosure, drawing and the appendedclaims.

-ln accordance with the present invention, l provide an improved processfor the polymerization of polymerizable hydrocarbons, for example,oletins polymerizable to normally solid polymers, in the presence of acatalyst which comprises first polymerizing at least -a portion `of thepolymerizable material in dilute phase in a streamline flow reactionzone, and then subjecting the partially polymerized admixture to furtherpolymerization in an agitated zone to substantially complete thepolymerization reaction.

More specifically, I provide an improved multistage process for thepolymerization of l-oleiins to normally solid polymers in the presenceof a catalyst which comprises continuously introducing olefin, diluentand catalyst into a small diameter highly elongated tubular reactionzone, a substantial portion of said reaction zone being surrounded by afluid heat exchange medium to remove heat of reaction, passing thereaction mixture under streamline flow conditions through said elongatedzone maintained at polymerization conditions of temperature Iandpressure to polymerize lat least a portion of said olen, passing theeilluent from said elongated zone to ECC an agitated tank-type reactionzone, preferably a series of stirred reactors maintained atpolymerization conditions of temperature and pressure to polymerizeunreacted olefin in said emuent, recovering polymer product from theagitated zone effluent, and recycling unreacted oleiin and diluent tosaid tubular Zone. Multipoint addition of recycled diluent provides foradditional temperature control in the elongated reaction zone and ispreferred.

Although :the invention is not limited to liquid-phase reaction, anadvantageous application thereof is to liquidphase operation, which is afrequently preferred mode of conducting the polymerization. Whenpolymerization is conducted in the liquid phase, it is often preferredto utilize the catalyst in the form of a slurry or suspension in aninert solvent or diluent. The invention is not limited to a particularcatalyst or diluent and any catalyst known for the polymerization ofolens to normally solid polymers can be advantageously employed in myprocess. Also, it should be understood that my invention is not limitedto any particular ydiluent or solvent for use in the process and thechoice of diluent will depend primarily upon the particular `catalystemployed or the type of polymer to be produced.

A particularly advantageous polymerization catalyst and diluent heat canbe employed in the process of my invention are described in U.S. Patent2,825,721 of Hogan et al. According to said patent, a chromium oxidecatalyst, preferably containing hexavalent chromium, associated with atleast one oxide selected from the group consisting of silica, alumina,zirconia and thoria and composites thereof is employed to preparenormally solid polymers of l-olens. Diluents or solvents that can beused in that process are hydrocarbon solvents which are inert and liquidunder the polymerization conditions such as paralns and naphthenessaving from 5 to 12 carbon atoms per molecule. Commonly knownpolymerization catalysts such as the Ziegler-type and the high-pressuretype polymerization catalysts as well as other diluents or solvents canalso be used, if desired, in my process.

The rst step of lthe present invention ycomprises introducing olefin,catalyst vand diluent into the inlet end of a small diameter highlyelongated tubular reaction zone, which is preferably surrounded by oneor more heat exchange jackets for removal of heat of reaction.Genorally, the iirst reaction zone will comprise a plurality of jacketedpipe loops positioned either horizontally or vertically. However, ifdesired a relatively long straight tubular reaction zone can beemployed. Multipoiut addition of cooled recycled `diluent aids heatremoval and is preferred. Also, the pipe loops can be individuallyjacketed so that ythey can be maintained at different temperatures asrequired to remove heat of reaction.

Olefin, catalyst, which is preferably suspended in diluent if inparticulate form, and diluent are introduced into the inlet end of thetubular reactor and passed therethrough under streamline or plug-like owconditions. The reaction at the inlet end of the tubular reactor takesplace in a very dilute polymer solution which has a correspondingly lowviscosity, and therefore heat removal is relatively easy in a simplejacket. Also, the presence of dilute solution in the :tubular reactormeans that lower jacket temperatures can be tolerated since there isless danger of -fouling the reactor walls with precipitated polymer. Asa given Volume of reaction mixture passes through the tubular reactormaintained under polymerization conditions of temperature and pressure,the mixture becomes more viscous due to continued polymerization of the:olefin to polymer and, therefore, removal of heat of reaction becomesincreasingly dicult since correspondingly lower heat transfercoefficients prevail. For any given flow rate and tube size there existsa limiting polymer concentration above which the available heat transfersurface of the tube wall could no longer be su'icient to dissipate theheat of reaction.

In accordance with the present invention, the partially polymerizedreaction mixture in the tubular reactor is passed directly to anagitated reaction zone, preferably a series of stirred reactors, alsomaintained under polymerization conditions to react additionalquantities of unreacted olefin present in the tubular reactor eluent.The polymerization reaction can be carried to completion in the agitatedreaction zone, if desired. The polymer concentration of the tubularreaction zone etiiuent will generally be below about l weight percent.The more viscous solution can be better handled and further polymerizedin the agitated tank reactor. Reaction conditions in the agitated zonecan be substantially the same as in the tubular reactor, as desired.

The operation of the tubular reactor of this invention in streamline owprevents the by-passing of catalyst that occurs in stirred reactor andin tubular reactors operated under conditions of turbulent i'low. In thelatter cases, substantial amounts of unused catalyst are removed vwiththe reactor euent. An important overall result of employing the reactionsystem of this invention is that much more ethcient use of the catalystis obtained and in many instances catalyst productivity is so high thatit is unnecessary to remove catalyst from the nal polymer product. Y

`Better understanding of the invention will be obtained upon referenceto the accompanying diagrammatic ow sheet which illustrates a preferredembodiment of the invention as applied to the polymerization ofethylene.

Referring now to the drawing, a liquid hydrocarbon solvent, such asnormal hexane or cyclohexane, is passed to reactor 11 by way of conduit15. Solvent in conduit is made up of recycled solvent introduced byconduit 13 and make-up solvent, as required, by way of conduit 14.Finely divided catalyst, for example, to 150 microns, is added to thesolvent in conduit 15 from feeder 16 to form a slurry or suspension ofthe catalyst in the solvent for introduction into reactor 11. Thecatalyst can comprise, for example, 2 weight percent chromium in theform of chromium oxide supported on a co-precipitated gel compositecomprising 90 weight percent silica and l0 weight percent alumina. Whenusing such a catalyst, the make-up solvent introduced into the system ispreferably pretreated to remove impurities such as dissolved Water,oxygen and sulfur compounds. The catalyst slurry is introduced into theinlet end of elongated reactor 11 and therein contacted with an olen,for example, ethylene, introduced by way of conduit 10.

Reactor 11 is an elongated tubular reactor and, as shown, is inconvolute form, but it is not outside the scope of the invention to havea relatively long straight tube. Reactor 11 has a small diameterrelative to the reactor length. Tubes of various diameters may be usedand the proper selection of flow velocity to maintain the reactants instreamline ow is well understood in the art. Elongated reactor 11 isshown in the drawing as being positioned vertically and comprises asingle vertical pipe loop. However, if desired, a plurality of such pipeloops can be employed positioned either horizontally or vertically.Vertical positioning of the tube assists polymer flow and is preferred.

Elongated reaction tube 11 is shown enclosed with individual heatexchange jackets 12 and 12a. Coolant is introduced into jackets 12 and12a by way of conduit 17. Warm coolant is removed from the jackets byway of conduits 18 and 18a, respectively, and then passed throughconduit 20. Control valves 19 and 19a regulate the ow of coolantWithdrawn from the jackets responsive to the temperature in each of thejackets. If desired, the individual jackets can be maintained atdifferent temperaltures as required. For example, when the reactor feedstreams enter the reactor below the desired reactor temperature, theheat exchange fluid passed through the rst jacket may be Warmer thansaid `feed streams, while the heat exchange fluid passed through thesubsequent jackets is cooler than the reaction mixture within thecorresponding zones of the reaction tube so that exotherrnic heat ofreaction can be removed.

As the reaction mixture passes through elongated reactor 11, olen ispolymerized to normally solid polymer. A rapid reaction takes place atthe inlet end of tubular reactor 11 in the dilute solution. As thesolution progresses through the reactor it increases in viscosity duetocontinued polymerization of oleiin. As the viscosity of the reactionmixture increases it becomes increasingly dicult to remove the heat ofreaction and, as shown in the drawing, cooled recycled solvent isadvantageously injected at a plurality of points along reaction tube 11to assist in the removal of the heat of reaction. The amount of recyclesolvent injected into the reactor is controlled by control valves 41, 44and 46 responsive to the temperature in the polymerization zone upstreamof the injection point as measured by temperature controllers 42, 43 and45, respectively.

The total eiuent removed from tubular reactor 11 is passed directly tostirred reactor 22 by way of conduit 21. Stirred reactor 22 is equippedywith an agitator 23 and heat exchange coil 47. Within stirred reactor22 additional unreacted olein present in tubular reactor 11 efduent ispolymerized. The resultant formation of increased quantities of polymerresults in an increase in the concentration of polymer in the reactionmixture in reactor 22, and consequently in an increase in the viscosityof said mixture so that as reaction proceeds the mixture becomesincreasingly dit`n`cult to handle and cool and, therefore, is maintainedunder agitated conditions. The reaction mixture is removed from reactor22 by Away of conduit 24 and introduced into stirred reactor 25 whereinadditional unreacted olefin is polymerized. Any number of stirredreactors desired in series can be employed in the present invention tocomplete the polymerization. Stirred reactor 25 is equipped with anagitator 26 and heat exchange coil 43.

The total reaction mixture of efuent from the system is removed fromstirred reactor 25 by Way of conduit 27 and contacted with cooledrecycled solvent introduced by conduit 38 to precipitate polymer in thesolution. The solution containing precipitated polymer is introducedinto holding tank 30 by way of conduit 28 having control valve 29.Polymer slurry is removed from tank 30 by way of conduit 31 and passedto a liquid-solid separation zone 32. As shown in the drawing,separation zone 32 is a cyclone separator. A high solids content slurryis removed from separator 32 and introduced into extruderdryer 33wherein polymer is recovered as a product of the process through conduit34. Solvent is removed from extruder-dryer 33 by way of conduit 35 andrecycled to reactor 11. Solvent and unreacted olen removed fromseparator 32 by way of conduit 36 is passed through cooler 37 whereinthe mixture is cooled to a temperature below about F. The eiiiuent fromcooler 37 is divided into two portions, one portion being sent throughconduit 38 to precipitate polymer in the reaction eluent in conduit 27,and the remainder recycled in conduit '39 to reactor 11. The recyclesolvent plus olefin present in conduit 39 is also divided, a portionbeing passed through conduits 13 and 15 to be mixed with catalyst, aspreviously described, and the remainder is injected at a plurality ofpoints lby conduit 40 into reactor 11 to assist control of reactiontemperature.

Reactor 11 can be of any desired length sucient to provide a reaction orresidence time therein of from several minutes to several hours. Thedesign and proportions of the pipe loops are generally well understoodby those skilled in the art. Also, any number of stirred reactors can beprovided to maintain a reaction or residence time therein sufficient tocomplete or substantially complete the polymerization. Also, it shouldbe understood that other arrangements of polymer recovery can beemployed comprising fractional distillation, filtration, evaporation,hashing and/or cooling equipment as is well known in the art.

Specific Example In a system of the type shown in the drawing 4500pounds of cyclohexane, 650 pounds of ethylene, 0.6 pound of catalyst arefed to the inlet end of elongated reactor 11 per day. The catalyst is iniinely divided form having a mean diameter of 60 microns, and is asupported chromium oxide-containing catalyst, such as is disclosed inHogan et al., supra. The ethylene and cyclohexane streams are preferablypuriiied to remove catalyst contaminants before introduction intoreactor 1i.

Reactor 11 is maintained at a temperature of 280 to 300 F. and atpressure suiiicient to maintain liquid phase conditions, generally 250to 500 p.s.i. Heat of reaction is removed by coolant circulated throughjackets 12 and 12a. 1940 pounds per day of cooled recycled solvent at atemperature of 250 F. are introduced into reactor 11 at a plurality ofpoints. The residence time of the catalyst in reactor 11 isapproximately 15 minutes. The linear velocity of liquid flow throughreactor 11 is about 5 yfeet per second.

Eiiiuent is withdrawn continuously from the outlet end of reactor 11 andpassed directly to stirred reactor 22. This eiuent comprises cyclohexanecontaining polyethylene dissolved therein, together with unreactedethylene and inert gas and catalyst suspended in the eiiluent. Theediuent withdrawn from reactor 11 contains approximately 3.7 weightpercent polyethylene, 5.5 weight percent unreacted ethylene, and 0.01weight percent of catalyst.

The temperature and pressure in reactors 22 and 25 are substantially thesame as in reactor 11. The average residence time in reactors 22 and 25is approximately 1 hour. The eiiiuent withdrawn from reactor 25 containsapproximately 7.3 weight percent polyethylene, and 1.9 weight percentunreacted ethylene. The efduent from reactor 25 is quenched with cooledrecycle cyclohexane having a temperature of about 100 F. to precipitatepolyethylene. The polymer slurry is passed to separator 32 for recoveryof polyethylene as product and solvent for recycle.

During the entire process, 515 pounds of polymer are produced each day.This represents a polyethylene yield of approximately 860 pounds perpound of catalyst. When the process is conducted in conventional stirredreactor systems, about 500 pounds of polyethylene per pound of catalystis produced. Thus, it is seen that the present invention in effectmaterially increases the polymer producing capacity of the catalyst.

While certain process steps, structures and example have been describedfor the purpose of illustration it is clear that the invention is notlimited thereto. Thus, while a specific example has been described withrespect to the production of polyethylene, similar results areobtainable when producing other olefin polymers, such as polypropylene,polybutene, copolymers of ethylene with propylene, butene and butadiene.The invention provides a process wherein a polymerizable hydrocarbon,for example, an oleiin, is polymerized in the presence of a solidcontact catalyst in dilute solution, iirst, in a small diameter, highlyelongated tubular reaction zone, and the reaction effluent from theelongated zone is subjected to further reaction at polymerizationconditions in an agitated zone in order to utilize more fully thecatalytic activity of the suspended catalyst. Variations and modicationsare possible within the scope of the foregoing disclosure and the claimsto the invention as will be apparent to those skilled in the art.

I claim:

1. A process `for obtaining high yields of normally solid polymer perunit weight of catalyst, which process comprises polymerizing an olefinin dilute phase in the presence of a catalyst and diluent undertemperature and pressure conditions suitable for the production ofnormally solid polymer in a rst reaction zone by passing the reactionmixture in streamline flow through an elongated tubular Zone, recoveringa crude reaction product mixture from said rst Zone containing up toabout 10 weight percent polymer, diluent, unreacted oleiin and catalyst,and passing said mixture to a second reaction zone comprising anagitated zone maintained under temperature and pressure conditionssuitable for the production of a normally solid polymer from saidolefin, forming such polymer in said zone, and recovering a polymerioproduct.

2. A process according to claim l wherein said oleiin is an aliphaticl-oleiin having a maximum chain length of 8 carbon atoms and nobranching nearer the double bond than the 4-position and said catalystcontains chromium oxide as an essential ingredient.

3. An improved process for the catalytic polymerization of oleiins toform normally solid polymers which comprises introducing a reactionmixture comprising an oleiin, an inert diluent and catalyst into anelongated tubular reaction zone, said reaction `zone being surrounded bya fluid heat exchange medium to remove heat of reaction, passing saidreaction mixture through said reaction zone in streamline flow underpolymerization conditions of temperature, pressure and residence timesuitable for the production of normally solid polymer in dilute phase,recovering a crude reaction product mixture from said elongated zonecontaining catalyst, diluent, unreacted olen and up to about l0 weightpercent solid polymer, passing said crude reaction mixture directly toan agitated reaction zone wherein further polymerization is eiected,separating polymer product and diluent from said agitated zone etiluent,and recovering said polymer as a product of the process.

4. A process according to claim 3 wherein said recovered diluent iscooled and then recycled to said tubular reaction zone to assist controlof reaction temperature.

5. Process according to claim 3 wherein said tubular reaction zonecomprises a plurality of upwardly extending pipe loops and said agitatedzone is a series of stirred reactors.

6. A process according to claim 5 wherein said oleiin is an aliphaticl-olen having a maximum chain length of 8 carbon atoms and no branchingnearer the double bond than the 4-position and said catalyst containschromium oxide as an essential ingredient.

7. An improved process for obtaining high yields of polyethylene perunit weight of catalyst which comprises polymerizing ethylene in adilute liquid phase in a small diameter highly elongated tubularreaction zone to form polyethylene from a portion of said ethylene bypassing said ethylene in streamline flow through said zone in thepresence of a particulate catalyst at a temperature in the range of to450 F. and in the presence of a liquid hydrocarbon solvent, saidreaction zone being surrounded by a fluid heat exchange medium to removeheat of reaction and maintain polymerization temperature within saidzone, recovering a crude reaction product from the exit end of saidtubular zone containing up to about 10 weight percent polyethylene,unreacted ethylene, and catalyst, passing said crude reaction mixturedirectly to an agitated tank-type reaction zone containing a pluralityof tanks in series, said agitated zone being maintained at a temperaturein the range of 150 to 450 F. and a pressure suicient to maintain liquidphase, reacting unreacted ethylene in the presence of catalyst to formadditional said polyethylene, recovering solvent and unreacted ethylenefrom the eiiuent of said agitated zone, and recovering said polyethylenefrom said agitated zone etiiuent as a product of the process.

8. Process according to claim 7 wherein said tubular reaction zonecomprises a Aplurality of upwardly extending pipe loops andv saidagitated Zone in a Series of stirred reactors. f Y

9. A process according to claim 7 wherein said recov ered solvent isrecycled to said tubular zone and is added Y afnormallysolid polymer, bycontacting said olefin in streamline ow in a small diameterhighlyelongated reaction zone with a catalystcomprising chromium oxide,including a substantial portion of hexavalent chromium,

' associated with at least one additional oxide selected from the groupconsisting of silica, alumina, zirconia and s y'thoria in thepre'senceof Va liquid hydrocarbon diluent under polymerization conditions,Vincluding a temperajture in the range of 150 F. to 450 F., suitable forthe polymerization of only a portion of Ysaidoleiin to a normally solidpolymer, said elongated zone being surrounded by a heat' exchange uid toremove the heat of reaction, passing theetiiuent'containing up to'about10 Weight percent polymer 'from said elongated zone directly to anagitated zone maintained under polymerizatien 'conditions to polymerizeadditional olen in said effluent and form a polymer solution lcontaininga substantial amount of polymer, said'polymerization in said agitatedzone being carried to jsubstantial completion, 'separating'polymer fromthe .euent from said agitated f zone, and recyclingrecovered" solvent tosaid'elongated zone'to absorb at least'a part of the heat of reaction.

References Cited in the file of this patent UNITED STATES PATENTS2,889,314 FritzY i June 2, 17959

1. A PROCESS FOR OBTAINING HIGH YIELDS OF NORMALLY SOLID POLYMER PERWEIGHT OF CATALYST, WHICH PROCESS COMPRISES POLYMERIING AN OLEFIN INDILUTE PHASE IN THE PRESENCE OF A CATALYST AND DILUTE UNDER TEMPERATUREAND PRESSURE CONDITIONS SUITABLE FOR THE PRODUCTION OF NORMALLY SOLIDPOLYMER IN A FIST REACTION ZONE BY PASSING THE REACTION MIXTURE INSTREAMLINE FLOW THROUGH AN ELONGATED TUBULAR ZONE, RECOVERING A CRUDEREACTION PRODUCT MIXTURE FROM SAID FIRST ZONE CONTAINING UP TO ABOUT 10WEIGHT PERCENT POLYMER, DILUENT, UNREACTED OLEFIN AND CATALYST, ANDPASSING SAID MIXTURE TO A SECOND REACTION ZONE COMPRISING AN AGITATEDZONE MAINTAINED UNDER TEMPERATURE AND PRESSURE CONDITIONS SUITABLE FORTHE, FORMTION OF A NORMALLY SOLID POLYMER FROM SAID OLEFIN, FORMING SUCHPOLYMER IN SAID ZONE, AND RECOVERING A POLYMERIC PRODUCT.